The reaction between hydrocarbon and steam produces a gas mixture comprising hydrogen, carbon monoxide, carbon dioxide and water vapor as well as typically some residual methane. Various processes are known for separating pure product from such mixtures. Some processes include an initial so-called shift reaction in which the carbon monoxide is converted to carbon dioxide. Such processes are unsuitable when carbon monoxide is desired as a product. These and other processes frequently employ cryogenic distillation in order to effect separation between hydrogen and the other constituents of the mixture after removal of carbon dioxide. However, cryogenic separation processes tend to have a high capital cost, particularly if more than one pure product is required.
The separation of hydrogen-rich gas mixtures, i.e. containing more than 50% by volume of hydrogen, by pressure swing adsorption (PSA) for separating hydrogen-rich gas mixtures is also well known. One such PSA cycle is disclosed in U.S. Pat. No. 3,430,418. In the cycle disclosed therein, the hydrogen-rich gas mixture is separated into hydrogen product and a waste gas stream. Many commercially practiced PSA processes utilize a similar cycle. They all have in common the feature of separating the incoming gas mixture into a hydrogen product stream and a single vent gas stream. The vent gas stream is, however, generally unsuitable for the production of carbon monoxide as its carbon monoxide content is relatively low.
A more elaborate PSA cycle for separating a gas mixture rich in hydrogen is described in European patent application 8882 A. The disclosed cycles are stated to be suitable for recovering separate hydrogen and methane products from a gas mixture comprising hydrogen, methane, and C.sub.2 or higher hydrocarbons. There is no suggestion of using the cycle to separate hydrogen and carbon monoxide products from a gas mixture comprising hydrogen, carbon monoxide and carbon dioxide, and hence there is no suggestion as to how the process might be integrated into a plant using a steam reformer to produce hydrogen and carbon monoxide products.
Another proposal for separating gas mixtures comprising hydrogen and two other components is disclosed in International patent application WO 86/05414. An example is given in this patent application of the separation of gas mixtures rich in hydrogen and carbon monoxide and with relatively low proportions of carbon dioxide (e.g. 1.5% by volume). There is no disclosure as to how such a process might be integrated into a plant for reforming hydrocarbon by reaction with steam. Moreover, the carbon dioxide concentrations from such a reformer are generally considerably higher than 1.5% by volume. In addition, the disclosed process withdraws both hydrogen and carbon monoxide-enriched gas from the same location. In practice, this makes it difficult to obtain a high purity hydrogen product.
There is thus a need for a noncryogenic method which makes possible the efficient production of relatively pure hydrogen and carbon monoxide products from a gas mixture formed by reforming hydrocarbon with steam. Such a need is not met by a process described in German patent application 3 427 804 A1 which discloses reforming hydrocarbon with carbon dioxide and then separating the resultant mixture into separate streams comprising carbon monoxide, hydrogen and carbon dioxide, but discloses no specific means for effecting this separation.